U.S. patent application number 15/324951 was filed with the patent office on 2017-07-27 for negative electrode active material for nonaqueous electrolyte secondary batteries, and nonaqueous electrolyte secondary battery.
This patent application is currently assigned to SANYO Electric Co., Ltd.. The applicant listed for this patent is SANYO Electric Co., Ltd.. Invention is credited to Atsushi Fukui, Yoshio Kato, Hiroshi Minami, Taizou Sunano, Kouhei Tuduki.
Application Number | 20170214041 15/324951 |
Document ID | / |
Family ID | 55439376 |
Filed Date | 2017-07-27 |
United States Patent
Application |
20170214041 |
Kind Code |
A1 |
Minami; Hiroshi ; et
al. |
July 27, 2017 |
NEGATIVE ELECTRODE ACTIVE MATERIAL FOR NONAQUEOUS ELECTROLYTE
SECONDARY BATTERIES, AND NONAQUEOUS ELECTROLYTE SECONDARY
BATTERY
Abstract
In a nonaqueous electrolyte secondary battery containing a
silicon material as a negative electrode active material, the
initial charge-discharge efficiency is improved. Negative electrode
active material particles (10) according to an embodiment each
contain a lithium silicate phase (11) represented by
Li.sub.2zSiO.sub.(2+z) (where 0<z<2) and silicon particles
(12) dispersed in the lithium silicate phase (11). In base
particles (13) each containing the lithium silicate phase (11) and
the silicon particles (12), preferably, a peak originating from
SiO.sub.2 is not observed at 2.theta.=25.degree. in an XRD pattern
obtained by XRD measurement of the particles.
Inventors: |
Minami; Hiroshi; (Hyogo,
JP) ; Tuduki; Kouhei; (Hyogo, JP) ; Fukui;
Atsushi; (Hyogo, JP) ; Sunano; Taizou;
(Tokushima, JP) ; Kato; Yoshio; (Osaka,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SANYO Electric Co., Ltd. |
Daito-shi, Osaka |
|
JP |
|
|
Assignee: |
SANYO Electric Co., Ltd.
Daito-shi, Osaka
JP
|
Family ID: |
55439376 |
Appl. No.: |
15/324951 |
Filed: |
August 27, 2015 |
PCT Filed: |
August 27, 2015 |
PCT NO: |
PCT/JP2015/004310 |
371 Date: |
January 9, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/485 20130101;
H01M 4/5825 20130101; H01M 4/364 20130101; H01M 2004/021 20130101;
H01M 4/386 20130101; Y02E 60/10 20130101; H01M 2004/027
20130101 |
International
Class: |
H01M 4/36 20060101
H01M004/36; H01M 4/58 20060101 H01M004/58; H01M 4/38 20060101
H01M004/38 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 3, 2014 |
JP |
2014-179344 |
Jan 28, 2015 |
JP |
2015-014158 |
Claims
1-9.
10. A negative electrode active material for a nonaqueous
electrolyte secondary battery, comprising: a lithium silicate phase
comprising Li.sub.2Si.sub.2O.sub.5 and silicon particles dispersed
in the lithium silicate phase.
11. The negative electrode active material for a nonaqueous
electrolyte secondary battery according to claim 10, wherein the
content of Li.sub.2Si.sub.2O.sub.5 is more than 50% by mass and
more with respect to the total mass of the lithium silicate
phase.
12. The negative electrode active material for a nonaqueous
electrolyte secondary battery according to claim 10, wherein the
content of Li.sub.2Si.sub.2O.sub.5 is more than 80% by mass and
more with respect to the total mass of the lithium silicate
phase.
13. The negative electrode active material for a nonaqueous
electrolyte secondary battery according to claim 10, wherein a
diffraction peak corresponding to a (111) plane of
Li.sub.2Si.sub.2O.sub.5 in the XRD pattern has a full width at half
maximum of 0.05.degree. or more.
14. The negative electrode active material for a nonaqueous
electrolyte secondary battery according to claim 10, wherein a
diffraction peak corresponding to a (111) plane of
Li.sub.2Si.sub.2O.sub.5 has a full width at half maximum of
0.09.degree. or more.
15. The negative electrode active material for a nonaqueous
electrolyte secondary battery according to claim 10, wherein no
peak originating from SiO.sub.2 is observed at 2.theta.=25.degree.
in an XRD pattern obtained by XRD measurement.
16. The negative electrode active material for a nonaqueous
electrolyte secondary battery according to claim 10, wherein the
lithium silicate phase does not include Li.sub.4SiO.sub.4.
17. The negative electrode active material for a nonaqueous
electrolyte secondary battery according to claim 16, wherein the
negative electrode active material does not include
Li.sub.4SiO.sub.4.
18. The negative electrode active material for a nonaqueous
electrolyte secondary battery according to claim 10, wherein
lithium silicate in the lithium silicate phase consists essentially
of Li.sub.2Si.sub.2O.sub.5.
19. The negative electrode active material for a nonaqueous
electrolyte secondary battery according to claim 10, wherein the
silicon particles are substantially uniformly dispersed form in the
lithium silicate phase.
20. The negative electrode active material for a nonaqueous
electrolyte secondary battery according to claim 10, wherein the
silicon particles have an average particle diameter of 200 nm or
less before initial charge.
21. The negative electrode active material for a nonaqueous
electrolyte secondary battery according to claim 10, wherein a
conductive layer is formed on a surface of a base particle
comprising the lithium silicate phase and the silicon
particles.
22. The negative electrode active material for a nonaqueous
electrolyte secondary battery according to claim 10, wherein the
content of Li.sub.2Si.sub.2O.sub.5 is more than 80% by mass and
more with respect to the total mass of the lithium silicate phase,
wherein a diffraction peak corresponding to a (111) plane of
Li.sub.2Si.sub.2O.sub.5 has a full width at half maximum of
0.09.degree. or more, wherein the negative electrode active
material does not include Li.sub.4SiO.sub.4.
23. The negative electrode active material for a nonaqueous
electrolyte secondary battery according to claim 22, wherein a
conductive layer is formed on a surface of a base particle
comprising the lithium silicate phase and the silicon
particles.
24. A nonaqueous electrolyte secondary battery comprising a
negative electrode containing the negative electrode active
material according to claim 10, a positive electrode, and a
nonaqueous electrolyte.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a negative electrode
active material for a nonaqueous electrolyte secondary battery, and
a nonaqueous electrolyte secondary battery.
BACKGROUND ART
[0002] Silicon materials, such as silicon (Si) and silicon oxides
represented by SiO.sub.x, are known to intercalate a large amount
of lithium ions per unit volume, compared with carbonaceous
material such as graphite. In particular, a change in the volume of
SiO.sub.x due to the intercalation of lithium ions is small,
compared with Si. Thus, the use of SiO.sub.x for negative
electrodes of lithium ion batteries and so forth has been studied.
For example, PTL 1 discloses a nonaqueous electrolyte secondary
battery including a negative electrode active material containing
SiO.sub.x mixed with graphite.
[0003] Nonaqueous electrolyte secondary batteries that contain
negative electrode active materials containing SiO.sub.x have a
disadvantage that the initial charge-discharge efficiency is low,
compared with the case where graphite is used as a negative
electrode active material. The main cause for this is that
SiO.sub.x is changed into Li.sub.4SiO.sub.4 (irreversible reaction
product) due to an irreversible reaction during charge and
discharge. To inhibit the irreversible reaction to improve the
initial charge-discharge efficiency, a negative electrode active
material represented by SiLi.sub.xO.sub.y (0<x<1.0,
0<y<1.5) is reported (see PTL 2). PTL 3 discloses a negative
electrode active material in which a lithium silicate phase
containing Li.sub.4SiO.sub.4 as a main component is contained in
silicon oxide.
CITATION LIST
Patent Literature
[0004] PTL 1: Japanese Published Unexamined Patent Application No.
2011-233245
[0005] PTL 2: Japanese Published Unexamined Patent Application No.
2003-160328
[0006] PTL 3: Japanese Published Unexamined Patent Application No.
2007-59213
SUMMARY OF INVENTION
Technical Problem
[0007] In each of the techniques disclosed in PTLs 2 and 3, the
initial charge-discharge efficiency is improved by heat-treating a
mixture of SiO.sub.x and a lithium compound at a high temperature
to convert SiO.sub.2 into Li.sub.4SiO.sub.4, which is an
irreversible reaction product, in advance. In this process,
however, SiO.sub.2 is left in particles, and Li.sub.4SiO.sub.4 is
formed only on surfaces of the particles. To perform the reaction
up to the inside of the particles, a higher-temperature process is
required. In that case, the grain size of each of Si and
Li.sub.4SiO.sub.4 is assumed to increase. For example, the increase
in grain size increases a change in the volume of active material
particles due to charge and discharge and reduces lithium-ion
conductivity.
Solution to Problem
[0008] According to an aspect of the present disclosure, a negative
electrode active material for a nonaqueous electrolyte secondary
battery includes a lithium silicate phase represented by
Li.sub.2zSiO.sub.(2+z) (where 0<z<2), and silicon particles
dispersed in the lithium silicate phase. In the negative electrode
active material for an nonaqueous electrolyte secondary battery, a
diffraction peak corresponding to a (111) plane of the lithium
silicate in an XRD pattern of the negative electrode active
material obtained by XRD measurement has a full width at half
maximum of 0.05.degree. or more.
Advantageous Effects of Invention
[0009] According to an aspect of the present disclosure, in a
nonaqueous electrolyte secondary battery containing a silicon
material as a negative electrode active material, the initial
charge-discharge efficiency can be improved.
BRIEF DESCRIPTION OF DRAWINGS
[0010] FIG. 1 is a cross-sectional view schematically illustrating
a negative electrode active material for a nonaqueous electrolyte
secondary battery according to an embodiment.
[0011] FIG. 2 illustrates an XRD pattern of a negative electrode
active material according to an embodiment (Example 1) for a
nonaqueous electrolyte secondary battery.
[0012] FIG. 3 illustrates an XRD pattern of a negative electrode
active material according to an embodiment (Example 9) for a
nonaqueous electrolyte secondary battery.
DESCRIPTION OF EMBODIMENTS
[0013] Embodiments will be described in detail below.
[0014] The drawings to which reference will be made in the
Description of Embodiments are schematically illustrated. For
example, the dimensional ratios of constituent elements in the
drawings are not always the same as those of the actual objects.
Specific dimensional ratios and other features are to be understood
from the description provided below.
[0015] A negative electrode active material according to an
embodiment of the present disclosure contains a lithium silicate
phase represented by Li.sub.2zSiO.sub.(2+z) (0<z<2) and
silicon particles dispersed in the phase. The silicon particles
preferably have an average particle diameter of 200 nm or less. The
negative electrode active material according to an embodiment of
the present disclosure may contain SiO.sub.2 on surfaces of the
silicon particles at a natural oxide film level. There are
significant differences in properties between SiO.sub.2 of natural
oxide films and SiO.sub.2 of conventional SiO.sub.x particles. For
example, in an XRD pattern obtained by XRD measurement of a
negative electrode active material according to an embodiment of
the present disclosure, a peak corresponding to SiO.sub.2 is not
observed at 2.theta.=25.degree.. The reason for this is presumably
that X-rays are not diffracted because the natural oxide film is
significantly thin. In contrast, in the XRD pattern of conventional
SiO.sub.x particles, a peak corresponding to SiO.sub.2 is observed
at 2.theta.=25.degree..
[0016] The conventional SiO.sub.x includes fine Si particles
dispersed in a SiO.sub.2 matrix. Reactions described below occur
during charge and discharge.
SiO.sub.x(2Si+2SiO.sub.2)+16Li.sup.++16e.sup.-.fwdarw.3Li.sub.4Si+Li.sub-
.4SiO.sub.4 (1)
[0017] Decomposition of formula 1 with regard to Si and 2SiO.sub.2
leads to the following formulae.
Si+4Li.sup.++4e.sup.-.fwdarw.Li.sub.4Si (2)
2SiO.sub.2+8Li.sup.++8e.sup.-.fwdarw.Li.sub.4Si+Li.sub.4SiO.sub.4
(3)
[0018] As described above, formula 3 represents an irreversible
reaction. The formation of Li.sub.4SiO.sub.4 is a main cause for a
reduction in initial charge-discharge efficiency.
[0019] The negative electrode active material according to an
embodiment of the present disclosure includes silicon particles
dispersed in the lithium silicate phase represented by
Li.sub.2zSiO.sub.(2+z) (0<z<2) and, for example, has a very
low SiO.sub.2 content, compared with the conventional SiO.sub.x.
The SiO.sub.2 contained in the negative electrode active material
is of the natural oxide films and has properties significantly
different from those of SiO.sub.2 of the conventional SiO.sub.x
particles. Thus, in the nonaqueous electrolyte secondary battery
containing the negative electrode active material, the reaction
represented by formula 3 seems to be less likely to occur to
improve the initial charge-discharge efficiency.
[0020] The negative electrode active material according to an
embodiment of the present disclosure has a particle structure in
which silicon particles having a small particle diameter are
dispersed in the lithium silicate phase. This reduces a change in
volume due to charge and discharge, thereby inhibiting the
breakdown of the particle structure. In the case where a
diffraction peak corresponding to the (111) plane of the lithium
silicate has a full width at half maximum of 0.05.degree. or more,
the lithium silicate phase has a structure similar to an amorphous
structure. This seemingly improves lithium-ion conductivity in
particles composed of the negative electrode active material to
reduce the change in volume due to charge and discharge. In the
negative electrode active material according to an embodiment of
the present disclosure, the change of the particle structure due to
charge and discharge is small, compared with the conventional
SiO.sub.x particles. In a nonaqueous electrolyte secondary battery
containing the negative electrode active material according to an
embodiment of the present disclosure, good initial charge-discharge
efficiency is obtained.
[0021] A nonaqueous electrolyte secondary battery according to an
embodiment includes a negative electrode containing the negative
electrode active material, a positive electrode, and a nonaqueous
electrolyte containing a nonaqueous solvent. A separator is
preferably arranged between the positive electrode and the negative
electrode. An example of the structure of the nonaqueous
electrolyte secondary battery is a structure including a case that
houses a nonaqueous electrolyte and an electrode assembly in which
a positive electrode and a negative electrode are wound with a
separator provided therebetween. A differently structured electrode
assembly, such as a stacked electrode assembly in which positive
electrodes and negative electrodes are stacked with separators
provided therebetween, may be used in place of the wound electrode
assembly. The nonaqueous electrolyte secondary battery may have any
form, for example, a cylinder form, a prism form, a coin form, a
button form, or a laminate form.
[Positive Electrode]
[0022] The positive electrode preferably includes a positive
electrode current collector formed of, for example, metal foil, and
a positive electrode mixture layer arranged on the current
collector. For the positive electrode current collector, foil
composed of a metal, such as aluminum, stable in the potential
range of the positive electrode, a film including a surface layer
composed of the metal, or the like may be used. The positive
electrode mixture layer preferably contains a conductive material
and a binder in addition to a positive electrode active material.
Particle surfaces of the positive electrode active material may be
covered with fine particles of an inorganic compound, for example,
an oxide, e.g., aluminum oxide (Al.sub.2O.sub.3), a phosphate
compound, or a borate compound.
[0023] Examples of the positive electrode active material include
lithium transition metal oxides containing transition metal
elements, such as Co, Mn, and Ni. Examples of the lithium
transition metal oxides include Li.sub.xCoO.sub.2,
Li.sub.xNiO.sub.2, Li.sub.xMnO.sub.2,
Li.sub.xCo.sub.yNi.sub.1-yO.sub.2,
Li.sub.xCo.sub.yM.sub.1-yO.sub.z, Li.sub.xNi.sub.1-yM.sub.yO.sub.z,
Li.sub.xMn.sub.2O.sub.4, Li.sub.xMn.sub.2-yM.sub.yO.sub.4,
LiMPO.sub.4, and Li.sub.2MPO.sub.4F (where M represents at least
one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and
B, 0<x.ltoreq.1.2, 0<y.ltoreq.0.9, 2.0.ltoreq.z.ltoreq.2.3).
These may be used separately or in combination as a mixture of two
or more.
[0024] The conductive material is used in order to increase the
electrical conductivity of the positive electrode mixture layer.
Examples of the conductive material include carbon materials, such
as carbon black, acetylene black, Ketjenblack, and graphite. These
may be used separately or in combination of two or more.
[0025] The binder is used in order to maintain a good contact state
between the positive electrode active material and the conductive
material and enhance the bondability of the positive electrode
active material and so forth to a surface of the positive electrode
current collector. Examples of the binder include fluorine-based
resins, such as polytetrafluoroethylene (PTFE) and polyvinylidene
fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins,
acrylic-based resins, and polyolefin-based resins. These resins may
be used in combination with carboxymethyl cellulose (CMC), its
salts (e.g., CMC-Na, CMC-K, and CMC-NH.sub.4; and partially
neutralized salts may also be usable), polyethylene oxide (PEO),
and so forth. These may be used separately or in combination of two
or more.
[Negative Electrode]
[0026] The negative electrode preferably includes a negative
electrode current collector formed of, for example, metal foil, and
a negative electrode mixture layer arranged on the current
collector. For the negative electrode current collector, foil
composed of a metal, such as copper, stable in the potential range
of the negative electrode, a film including a surface layer
composed of the metal, or the like may be used. The negative
electrode mixture layer preferably contains a binder in addition to
the negative electrode active material. As with the case of the
positive electrode, examples of the binder that may be used include
fluorine-based resins, PAN, polyimide-based resins, acrylic-based
resins, and polyolefin-based resins. When a mixture slurry is
prepared with an aqueous solvent, CMC or its salt (e.g., CMC-Na,
CMC-K, or CMC-NH.sub.4; or a partially neutralized salt may be
used), styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or
its salt (e.g., PAA-Na or PAA-K; or a partially neutralized salt
may be used), or polyvinyl alcohol (PVA) may be preferably
used.
[0027] FIG. 1 is a cross-sectional view of one of negative
electrode active material particles 10 according to an
embodiment.
[0028] As illustrated in FIG. 1, each of the negative electrode
active material particles 10 contains a lithium silicate phase 11
and silicon particles 12 dispersed in the phase. Preferably, the
negative electrode active material particles 10 contain SiO.sub.2
at a natural oxide film level, and a peak corresponding to
SiO.sub.2 is not observed at 2.theta.=25.degree. in an XRD pattern
obtained by XRD measurement of the negative electrode active
material particles 10. A conductive layer 14 is preferably arranged
on a surface of a base particle 13 containing the lithium silicate
phase 11 and the silicon particles 12.
[0029] The base particle 13 may contain a third component other
than the lithium silicate phase 11 or the silicon particles 12.
When the base particle 13 contains SiO.sub.2 of a natural oxide
film, the base particle 13 preferably has a SiO.sub.2 content less
than 10% by mass and more preferably less than 7% by mass. A
smaller particle diameter of the silicon particles 12 results in a
larger surface area and thus a larger amount of SiO.sub.2 of the
natural oxide film.
[0030] The silicon particles 12 in the negative electrode active
material particles 10 can intercalate a large amount of lithium
ions, compared with those of carbon materials such as graphite.
Thus, the use of the negative electrode active material particles
10 as a negative electrode active material contributes to an
increase in the capacity of a battery. The negative electrode
active material particles 10 may be used alone as the negative
electrode active material in the negative electrode mixture layer.
However, a change in the volume of the silicon material due to
charge and discharge is larger than that of graphite. Thus, in
order to achieve higher capacity and maintain good cycle
characteristics, another active material that exhibits only a small
change in volume may be used in combination. As the another active
material, a carbon material such as graphite is preferred.
[0031] Examples of the graphite that may be used include graphites
that have been used as negative electrode active materials, for
example, natural graphites, such as flake graphite, massive
graphite, and earthy graphite, and artificial graphites, such as
massive artificial graphite (MAG) and graphitized mesophase carbon
microbeads (MCMB). In the case where graphite is used in
combination, the ratio by mass of the negative electrode active
material particle 10 to graphite is preferably 1:99 to 30:70. When
the ratio of the negative electrode active material particles 10 to
graphite is in the range, both of an increase in capacity and
improvement in cycle characteristics are easily achieved. When the
ratio of the negative electrode active material particles 10 to
graphite is less than 1% by mass, the increase in capacity by the
addition of the negative electrode active material particles 10 is
not so advantageous.
[0032] In the negative electrode active material particles 10 (base
particles 13), preferably, the fine silicon particles 12 are
substantially uniformly dispersed in the lithium silicate phase 11.
For example, each of the base particles 13 has a sea-island
structure in which fine Si is dispersed in a lithium silicate
matrix and Si is substantially uniformly dispersed in any cross
section without being localized in a region. The content of the
silicon particles 12 (Si) in the base particles 13 is preferably
20% to 95% by mass and more preferably 35% to 75% by mass with
respect to the total mass of the base particles 13 from the
viewpoints of increasing the capacity and improving the cycle
characteristics. An excessively low content of Si leads to, for
example, a reduction in charge-discharge capacity and the
degradation of load characteristics due to insufficient diffusion
of lithium ions. At an excessively high content of Si, for example,
part of Si is exposed without being covered with a lithium silicate
and thus comes into contact with an electrolytic solution to
degrade the cycle characteristics.
[0033] The lithium silicate phase 11 and the silicon particles 12
are preferably formed of fine particles. The lithium silicate phase
11 is formed of, for example, finer particles than the silicon
particles 12. FIG. 2 illustrates an XRD pattern of the base
particles 13 before charge and discharge. In the XRD pattern of
FIG. 2, the intensities of peaks originating from the lithium
silicate and Si are weak, and the peaks originating from the
lithium silicate are smaller than peaks originating from Si.
Regarding the base particles 13, the peaks originating from the
lithium silicate are preferably smaller than the peaks originating
from Si in the XRD pattern. In the XRD pattern of the negative
electrode active material particles 10, for example, the intensity
of a diffraction peak corresponding to the (111) plane of Si is
greater than that of a diffraction peak corresponding to the (111)
plane of the lithium silicate.
[0034] The lithium silicate phase 11 is composed of a lithium
silicate represented by Li.sub.2zSiO.sub.(2+z) (0<z<2). That
is, the lithium silicate contained in the lithium silicate phase 11
does not include Li.sub.4SiO.sub.4 (Z=2). Li.sub.4SiO.sub.4 is an
unstable compound and reacts with water to be alkaline. This alters
Si to reduce the charge-discharge capacity. The lithium silicate
phase 11 is preferably composed of Li.sub.2SiO.sub.3 (Z=1) or
Li.sub.2Si.sub.2O.sub.5 (Z=1/2) serving as a main component
(component whose proportion by mass is highest) in view of
stability, ease of production, lithium-ion conductivity, and so
forth. In the case where Li.sub.2SiO.sub.3 or
Li.sub.2Si.sub.2O.sub.5 is used as a main component, the content of
the main component is preferably more than 50% by mass and more
preferably 80% by mass or more with respect to the total mass of
the lithium silicate phase 11.
[0035] The silicon particles 12 have an average particle diameter
of, for example, 500 nm or less, preferably 200 nm or less, and
more preferably 50 nm or less before initial charge. After charge
and discharge, the silicon particles 12 preferably have an average
particle diameter of 400 nm or less and more preferably 100 nm or
less. A reduction in the size of the silicon particles 12 reduces a
change in volume during charge and discharge, thereby easily
inhibiting the breakdown of an electrode structure. The average
particle diameter of the silicon particles 12 is measured by
observation of cross sections of the negative electrode active
material particles 10 with a SEM or TEM. Specifically, the average
particle diameter of the silicon particles 12 is determined by
averaging the maximum diameters of 100 particles selected from the
silicon particles 12.
[0036] In the XRD pattern obtained by XRD measurement of the
negative electrode active material particles 10 (base particles
13), a diffraction peak corresponding to the (111) plane of the
lithium silicate has a full width at half maximum of 0.05.degree.
or more. As described above, the full width at half maximum is
adjusted to 0.05.degree. or more to reduce the crystallinity of the
lithium silicate phase. This seemingly improves lithium-ion
conductivity in the particle to reduce the change in the volume of
the silicon particles 12 due to charge and discharge. The full
width at half maximum of the diffraction peak corresponding to the
(111) plane of the lithium silicate varies slightly, depending on
the component of the lithium silicate phase 11, and is preferably
0.09.degree. or more, for example, in the range of 0.09.degree. to
0.55.degree..
[0037] The full width at half maximum of the diffraction peak
corresponding to the (111) plane of the lithium silicate is
measured under conditions described below. In the case where
multiple types of lithium silicates are contained, the full width
at half maximum (.degree. (2.theta.)) of a diffraction peak
corresponding to the (111) plane of each of the multiple types of
lithium silicates is measured. In the case where a diffraction peak
corresponding to the (111) plane of a lithium silicate overlaps a
diffraction peak corresponding to different plane indices or a
diffraction peak originating from another substance, the
diffraction peak corresponding to the (111) plane of the lithium
silicate is isolated, and then the full width at half maximum is
measured. Measurement apparatus: X-ray diffractometer (Model:
RINT-TTRII), manufactured by Rigaku Corporation
Anticathode: Cu
[0038] Tube voltage: 50 kV Tube current: 300 mA Optical system:
collimated beam system [Incident side: multilayer mirror
(divergence angle: 0.05.degree., beam width: 1 mm), Soller slit
(5.degree.); Receiving side: long slit PSA200 (resolution:
0.057.degree.), Soller slit)(5.degree. ] Step width: 0.01.degree.
or 0.02.degree. Counting time: 1 to 6 s
[0039] In the case where the lithium silicate phase 11 is composed
of Li.sub.2Si.sub.2O.sub.5 serving as a main component, a
diffraction peak corresponding to the (111) plane of
Li.sub.2Si.sub.2O.sub.5 in an XRD pattern of the negative electrode
active material particles 10 preferably has a full width at half
maximum of 0.09.degree. or more. For example, in the case where
Li.sub.2Si.sub.2O.sub.5 accounts for 80% by mass or more of the
total mass of the lithium silicate phase 11, an example of a
preferred full width at half maximum of the diffraction peak is in
the range of 0.09.degree. to 0.55.degree.. In the case where the
lithium silicate phase 11 is composed of Li.sub.2SiO.sub.3 serving
as a main component, a diffraction peak corresponding to the (111)
plane of Li.sub.2SiO.sub.3 in an XRD pattern of the negative
electrode active material particles 10 preferably has a full width
at half maximum of 0.10.degree. or more. For example, in the case
where Li.sub.2SiO.sub.3 accounts for 80% by mass or more of the
total mass of the lithium silicate phase 11, an example of a
preferred full width at half maximum of the diffraction peak is in
the range of 0.10.degree. to 0.55.degree..
[0040] The negative electrode active material particles 10
preferably have an average particle diameter of 1 to 15 .mu.m and
more preferably 4 to 10 .mu.m from the viewpoint of, for example,
increasing the capacity and improving the cycle characteristics.
Here, the average particle diameter of the negative electrode
active material particles 10 indicates a particle diameter (volume
mean diameter) at an accumulated volume of 50% of a particle size
distribution of primary particles measured by a laser
diffraction/scattering method (for example, with "LA-750"
manufactured by HORIBA, Ltd). An excessively small average particle
diameter of the negative electrode active material particles 10
results in a large surface area; hence, the amount of the negative
electrode active material particles 10 reacting with the
electrolyte tends to increase, thereby reducing the capacity. An
excessively large average particle diameter results in an increase
in the amount of volume change due to charge and discharge; hence,
the cycle characteristics tend to degrade. The conductive layer 14
is preferably arranged on a surface of each of the negative
electrode active material particles 10 (base particles 13).
However, the conductive layer 14 does not affect the average
particle diameter of the negative electrode active material
particles 10 because of the small thickness of the conductive layer
14 (particle diameter of the negative electrode active material
particles 10.apprxeq.particle diameter of the base particles
13).
[0041] The base particles 13 are formed through, for example, the
following steps 1 to 3.
(1) Si and a lithium silicate are mixed together in a ratio by mass
of 20:80 to 95:5 to prepare a mixture. (2) The mixture is
pulverized into fine particles with a ball mill. A mixture may be
prepared by reducing the size of a powder of each of the raw
materials and mixing the resulting powders together. (3) The
pulverized mixture is subjected to, for example, heat treatment in
an inert atmosphere at 600.degree. C. to 1000.degree. C. In the
heat treatment, a pressure may be applied to form a sintered
member, like hot pressing. In this case, the sintered member is
pulverized into particles having a predetermined particle diameter.
The lithium silicate represented by Li.sub.2zSiO.sub.(2+z)
(0<z<2) is stable in the temperature range and does not react
with Si; hence, the capacity is not reduced. The base particles 13
may be produced by synthesizing Si nanoparticles and lithium
silicate nanoparticles without using a ball mill, mixing these
nanoparticles together, and performing heat treatment.
[0042] Each of the negative electrode active material particles 10
preferably includes the conductive layer 14 on the particle
surface, the conductive layer 14 being composed of a material with
higher conductivity than the lithium silicate phase 11 and the
silicon particles 12. A conductive material contained in the
conductive layer 14 is preferably electrochemically stable and is
preferably at least one selected from the group consisting of
carbon materials, metals, and metal compounds. As the carbon
materials, carbon black, acetylene black, Ketjenblack, and
graphite, and mixtures of two or more of them may be used, as with
the conductive materials for the positive electrode mixture layer.
As the metals, copper, nickel, and alloys thereof, which are stable
in a potential range of the negative electrode, may be used. As the
metal compounds, copper compounds, nickel compounds, and so forth
may be exemplified (layers of the metal or metal compound may be
formed on surfaces of the base particles 13 by, for example,
electroless plating). Among these, the carbon materials are
particularly preferably used.
[0043] Examples of a method for coating the surfaces of the base
particles 13 with carbon include a CVD method with acetylene,
methane, or the like; and a method in which coal pitch, petroleum
pitch, a phenolic resin, or the like is mixed with the base
particles 13 and then heat treatment is performed. Carbon coating
layers may be formed by fixing carbon black, Ketjenblack, or the
like to the surfaces of the base particles 13 with a binder.
[0044] Each of the conductive layers 14 is preferably arranged so
as to cover substantially the whole of the region of the surface of
a corresponding one of the base particles 13. Each conductive layer
14 preferably has a thickness of 1 to 200 nm and more preferably 5
to 100 nm from the viewpoint of achieving good conductivity and the
diffusibility of lithium ions into the base particles 13. An
excessively small thickness of the conductive layer 14 results in
low conductivity and makes it difficult to uniformly cover each of
the base particles 13. An excessive large thickness of the
conductive layer 14 has a tendency to inhibit the diffusion of
lithium ions into the base particle 13 to reduce the capacity. The
thickness of the conductive layer 14 may be measured by
cross-sectional observation of particles with, for example, a SEM
or TEM.
[Nonaqueous Electrolyte]
[0045] The nonaqueous electrolyte contains a nonaqueous solvent and
an electrolyte salt dissolved in the nonaqueous solvent. The
nonaqueous electrolyte is not limited to a liquid electrolyte
(nonaqueous electrolyte solution) and may be a solid electrolyte
containing a gel-like polymer or the like. Examples of the
nonaqueous solvent that may be used include esters, ethers,
nitriles such as acetonitrile, amides such as dimethylformamide,
and solvent mixtures of two or more of them. The nonaqueous solvent
may contain a halogen-substituted solvent in which at least one
hydrogen atom of each of the solvents is replaced with a halogen
atom such as a fluorine atom.
[0046] Examples of the esters include cyclic carbonates, such as
ethylene carbonate (EC), propylene carbonate (PC), and butylene
carbonate; chain carbonates, such as dimethyl carbonate (DMC),
methyl ethyl carbonate (EMC), diethyl carbonate (DEC), methyl
propyl carbonate, ethyl propyl carbonate, and methyl isopropyl
carbonate; cyclic carboxylates, such as .gamma.-butyrolactone (GBL)
and .gamma.-valerolactone (GVL); chain carboxylates, such as methyl
acetate, ethyl acetate, propyl acetate, methyl propionate (MP),
ethyl propionate, and .gamma.-butyrolactone.
[0047] Examples of the ethers include cyclic ethers, such as
1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran,
2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide,
1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran,
1,8-cineole, and crown ethers; and chain ethers, such as
1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl
ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl
ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether,
pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl
ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane,
1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene
glycol diethyl ether, diethylene glycol dibutyl ether,
1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol
dimethyl ether, and tetraethylene glycol dimethyl.
[0048] As the halogen-substituted solvent, a fluorinated cyclic
carbonate such as fluoroethylene carbonate (FEC), a fluorinated
chain carbonate, a fluorinated chain carboxylate such as
fluoromethyl propionate (FMP), or the like is preferably used.
[0049] The electrolyte salt is preferably a lithium salt. Examples
of the lithium salt include LiBF.sub.4, LiClO.sub.4, LiPF.sub.6,
LiAsF.sub.6, LiSbF.sub.6, LiAlCl.sub.4, LiSCN, LiCF.sub.3SO.sub.3,
LiCF.sub.3CO.sub.2, Li(P(C.sub.2O.sub.4)F.sub.4),
LiPF.sub.6-x(C.sub.nF.sub.2n+1).sub.x (where 1<x<6, and n
represents 1 or 2), LiB.sub.10Cl.sub.10, LiCl, LiBr, LiI,
chloroborane lithium, lower aliphatic lithium carboxylates,
borates, such as Li.sub.2B.sub.4O.sub.7 and
Li(B(C.sub.2O.sub.4)F.sub.2), and imide salts, such as
LiN(SO.sub.2CF.sub.3).sub.2 and
LiN(C.sub.lF.sub.2l+1SO.sub.2)(C.sub.mF.sub.2m+1SO.sub.2) {where l
and m each represent an integer of 1 or more}. These lithium salts
may be used separately or in combination as a mixture of two or
more of them. Among these, LiPF.sub.6 is preferably used in view of
ionic conductivity and electrochemical stability. The concentration
of the lithium salt is preferably 0.8 to 1.8 mol per liter of the
nonaqueous solvent.
[Separator]
[0050] As the separator, a porous sheet having ion permeability and
insulating properties is used. Specific examples of the porous
sheet include fine porous thin membranes, woven fabrics, and
nonwoven fabrics. Preferred examples of a material of the separator
include olefin-based resins, such as polyethylene and
polypropylene; and cellulose. The separator may be formed of a
laminated body including a cellulose fiber layer and a
thermoplastic resin fiber layer composed of an olefin-based resin
or the like.
EXAMPLES
[0051] While the present invention will be further described below
by examples, the present invention is not limited to these
examples.
Example 1
[Production of Negative Electrode Active Material]
[0052] Si (three nines (3 N), pulverized to 10 .mu.m) and
Li.sub.2SiO.sub.3 (pulverized to 10 .mu.m) were mixed together in a
ratio by mass of 50:50 in an inert atmosphere and charged into a
pot (composed of stainless steel (SUS), volume: 500 mL) of a
planetary ball mill (P-5, manufactured by Fritsch). Into the pot,
24 balls (diameter: 20 mm) composed of stainless steel (SUS) were
charged. A lid was closed. Pulverization treatment was performed at
200 rpm for 50 hours. Then the resulting powder was removed in an
inert atmosphere and subjected to heat treatment in an inert gas
atmosphere at 800.degree. C. for 4 hours. The heat-treated powder
(hereinafter, referred to as "base particles") was pulverized. The
pulverized powder was passed through a mesh with 40-.mu.m openings,
mixed with coal pitch (MCP250, manufactured by JFE Chemical
Corporation), and subjected to heat treatment in an inert
atmosphere at 800.degree. C. to coat surfaces of the base particles
with carbon, thereby forming conductive layers. The coating weight
of carbon is 5% by mass with respect to the total mass of the
particles including the base particles and the conductive layers.
The average particle diameter of the resulting particles was
adjusted to 5 .mu.m with a sieve to produce negative electrode
active material A1.
[Analysis of Negative Electrode Active Material]
[0053] TEM observation of a cross section of negative electrode
active material A1 revealed that the Si particles had an average
particle diameter less than 50 nm. SEM observation of the cross
section of negative electrode active material A1 revealed that the
Si particles were substantially uniformly dispersed in a matrix
composed of Li.sub.2SiO.sub.3. In an XRD pattern of negative
electrode active material A1 (see FIG. 2), peaks originating mainly
from Si and Li.sub.2SiO.sub.3 were observed. The intensities of
these peaks were as follows: Si>Li.sub.2SiO.sub.3. No peak
originating from SiO.sub.2 was observed at 2.theta.=25.degree..
Measurement of negative electrode active material A1 by Si-NMR
revealed that the content of SiO.sub.2 was less than 7% by mass
(equal to or lower than the minimum limit of detection).
[Production of Negative Electrode]
[0054] The negative electrode active material and polyacrylonitrile
(PAN) were mixed together in a ratio by mass of 95:5. After
addition of N-methyl-2-pyrrolidone (NMP), the resulting mixture was
stirred with a mixer ("AWATORI RENTARO" (Thinky Mixer),
manufactured by Thinky Corporation) to prepare a negative electrode
mixture slurry. The slurry was applied to a surface of copper foil
in such a manner that the mass of a negative electrode mixture
layer was 25 g per square meter. The resulting coating film was
dried in air at 105.degree. C. and subjected to rolling to produce
a negative electrode. The negative electrode mixture layer had a
packing density of 1.50 g/cm.sup.3.
[Preparation of Nonaqueous Electrolytic Solution]
[0055] LiPF.sub.6 was added to a solvent mixture of ethylene
carbonate (EC) and diethyl carbonate (DEC) mixed in a ratio by
volume of 3:7 to prepare a nonaqueous electrolytic solution having
a concentration of 1.0 mol/L.
[Production of Nonaqueous Electrolyte Secondary Battery]
[0056] Lithium metal foil and the negative electrode provided with
a Ni tab were arranged in an inert atmosphere so as to face each
other with a polyethylene separator interposed therebetween,
thereby forming an electrode assembly. The electrode assembly was
placed in a battery case formed of an aluminum laminated film. The
nonaqueous electrolytic solution was injected into the battery
case. The battery case was sealed, thereby producing battery
T1.
Example 2
[0057] Negative electrode active material A2 and battery T2 were
produced in the same way as in Example 1, except that the treatment
time with the ball mill was changed to 200 hours. The Si particles
had an average particle diameter less than 10 nm.
Example 3
[0058] Negative electrode active material A3 and battery T3 were
produced in the same way as in Example 1, except that the treatment
time with the ball mill was changed to 10 hours. The Si particles
had an average particle diameter less than 200 nm.
Example 4
[0059] Negative electrode active material A4 and battery T4 were
produced in the same way as in Example 1, except that the treatment
time with the ball mill was changed to 2 hours. The Si particles
had an average particle diameter less than 500 nm.
Example 5
[0060] Negative electrode active material A5 and battery T5 were
produced in the same way as in Example 1, except that
Li.sub.2Si.sub.2O.sub.5 was used in place of Li.sub.2SiO.sub.3. In
an XRD pattern of negative electrode active material A5, peaks
originating from Si and Li.sub.2Si.sub.2O.sub.5 were observed. A
peak originating from Li.sub.2SiO.sub.3 was also observed. The
intensities of these peaks were as follows:
Si>Li.sub.2Si.sub.2O.sub.5>Li.sub.2SiO.sub.3.
Comparative Example 1
[0061] Si (three nines (3 N), pulverized to 10 .mu.m) and
Li.sub.2SiO.sub.3 (pulverized to 10 .mu.m) were each pulverized in
an inert atmosphere for 50 hours with the ball mill and then were
mixed together in a ratio by mass of 50:50. The resulting mixture
was directly used as negative electrode active material B1 without
heat treatment. Battery R1 was produced in the same way as in
Example 1. In negative electrode active material B1, while
Li.sub.2SiO.sub.3 particles adhered to surfaces of the Si
particles, a Li.sub.2SiO.sub.3 matrix (continuous phase) was not
formed. That is, negative electrode active material B1 did not have
a composite particle structure in which the Si particles were
dispersed in the Li.sub.2SiO.sub.3 phase.
Comparative Example 2
[0062] Battery R2 was produced in the same way as in Example 1,
except that SiO.sub.x coated with carbon layers was used as
negative electrode active material B2, the SiO.sub.x coated with
carbon layers being produced by mixing SiO.sub.x (where x=0.97, and
average particle diameter: 5 .mu.m) with the coal pitch and
subjecting the mixture to heat treatment in an inert atmosphere at
800.degree. C.
[0063] Regarding the batteries of Examples 1 to 5 and Comparative
examples 1 and 2, the initial charge-discharge efficiency was
evaluated by a method described below. Table 1 lists the evaluation
results.
[Initial Charge-Discharge Efficiency]
[0064] Charge
[0065] Constant-current charge was performed at a current of 0.2 It
until the voltage reached 0 V. Subsequently, constant-current
charge was performed at a current of 0.05 It until the voltage
reached 0 V.
[0066] Discharge
[0067] Constant-current discharge was performed at a current of 0.2
It until the voltage reached 1.0 V.
[0068] Interval
[0069] An interval between the charge and the discharge was 10
minutes.
[0070] The ratio of discharge capacity to charge capacity at the
first cycle was defined as initial charge-discharge efficiency.
Initial charge-discharge efficiency (%)=discharge capacity at first
cycle/charge capacity at first cycle.times.100
TABLE-US-00001 TABLE 1 Average par- Initial charge- ticle diam-
Lithium Structure discharge eter of Si silicate of particles
efficiency T1 <50 nm Li.sub.2SiO.sub.3 composite particles 85%
T2 <10 nm 87% T3 <200 nm 82% T4 <500 nm 78% T5 <50 nm
Li.sub.2Si.sub.2O.sub.5 85% R1 <50 nm Li.sub.2SiO.sub.3 mixed
particles 71% R2 -- -- composite particles 68%
[0071] As listed in Table 1, each of batteries T1 to T5 of these
examples has good initial charge-discharge efficiency, compared
with batteries R1 and R2 of these comparative examples. In other
words, the use of the composite particles for a negative electrode
active material, the composite particles each containing the Si
particles dispersed in the Li.sub.2SiO.sub.3 or
Li.sub.2Si.sub.2O.sub.5 matrix, improves the initial
charge-discharge efficiency, compared with the case where the Si
particles and Li.sub.2SiO.sub.3 particles are merely mixed together
or where SiO.sub.x is used. In each of the batteries of these
examples, a smaller average particle diameter of the Si particles
resulted in better initial charge-discharge efficiency. The main
cause for the results is presumably that the change in volume due
to charge and discharge decreased with decreasing diameter of the
Si particles.
Example 6
[Production of Positive Electrode]
[0072] Lithium cobaltate, acetylene black (HS100, manufactured by
Denki Kagaku Kogyo K.K.), and polyvinylidene fluoride (PVdF) were
mixed together in a ratio by mass of 95:2.5:2.5. After
N-methyl-2-pyrrolidone (NMP) serving as a dispersion medium was
added to the resulting mixture, the mixture was stirred with a
mixer (T.K. HIVIS MIX, manufactured by PRIMIX Corporation) to
prepare a positive electrode mixture slurry. The positive electrode
mixture slurry was applied to aluminum foil, dried, subjected to
rolling with reduction rolls, thereby producing a positive
electrode including a positive electrode mixture layer formed on
each surface of the aluminum foil, the positive electrode mixture
layer having a density of 3.6 g/cm.sup.3.
[Production of Negative Electrode]
[0073] Negative electrode active material A1 used in Example 1 and
graphite were mixed together in a ratio by mass of 5:95. The
resulting mixture was used as negative electrode active material A6
(negative electrode active material A1: 5% by mass). Negative
electrode active material A6, sodium carboxymethyl cellulose
(CMC-Na), and styrene-butadiene rubber (SBR) were mixed together in
a ratio by mass of 97.5:1.0:1.5. Water was added thereto. The
resulting mixture was stirred with a mixer (T.K. HIVIS MIX,
manufactured by PRIMIX Corporation) to prepare a negative electrode
mixture slurry. The slurry was applied to copper foil in such a
manner that the mass of a negative electrode mixture layer was 190
g per square meter. The resulting coating film was dried in air at
105.degree. C. and subjected to rolling to produce a negative
electrode including the negative electrode mixture layer formed on
each surface of the copper foil, the negative electrode mixture
layer having a density of 1.5 g/cm.sup.3.
[Production of Nonaqueous Electrolyte Secondary Battery]
[0074] Tabs were attached to the electrodes. The positive electrode
and the negative electrode equipped with the tabs were spirally
wound with a separator interposed therebetween in such a manner
that the tabs were located at the outermost peripheral portion. The
resulting electrode assembly was inserted into a case formed of an
aluminum laminated sheet. After vacuum drying was performed at
105.degree. C. for 2 hours, the nonaqueous electrolytic solution
was injected. An opening portion of the case was sealed, thereby
producing battery T6. The design capacity of the battery was 800
mAh.
Example 7
[0075] Negative electrode active material A7 and battery T7 were
produced in the same way as in Example 6, except that the amount of
negative electrode active material A1 added was changed to 10% by
mass.
Example 8
[0076] Negative electrode active material A8 and battery T8 were
produced in the same way as in Example 6, except that the amount of
negative electrode active material A1 added was changed to 30% by
mass.
Comparative Example 3
[0077] Negative electrode active material B3 and battery R3 were
produced in the same way as in Example 6, except that negative
electrode active material B2 used in Comparative example 2 was used
in place of negative electrode active material A1.
Comparative Example 4
[0078] Negative electrode active material B4 and battery R4 were
produced in the same way as in Example 6, except that negative
electrode active material B2 was used in place of negative
electrode active material A1.
Comparative Example 5
[0079] Negative electrode active material B5 and battery R5 were
produced in the same way as in Example 6, except that negative
electrode active material B2 was used in place of negative
electrode active material A1.
[0080] Regarding the batteries of Examples 6 to 8 and Comparative
examples 3 to 5, the initial charge-discharge efficiency and the
charge-discharge cycle characteristics were evaluated by methods
described below. Table 2 lists the evaluation results.
[Initial Charge-Discharge Efficiency]
[0081] Charge
[0082] Constant-current charge was performed at a current of 1 It
(800 mA) until the voltage reached 4.2 V. Subsequently,
voltage-constant charge was performed at a voltage of 4.2 V until
the current reached 1/20 It (40 mA).
[0083] Discharge
[0084] Constant-current discharge was performed at a current of 1
It (800 mA) until the voltage reached 2.75 V.
[0085] Interval
[0086] An interval between the charge and the discharge was 10
minutes.
[0087] The initial charge-discharge efficiency of each of the
batteries was measured under the charge-discharge conditions.
[Cycle Test]
[0088] Each of the batteries was subjected to a cycle test under
the charge-discharge conditions. The number of cycles required to
cause the discharge capacity to reach 80% of the discharge capacity
at the first cycle was measured and was defined as a cycle life.
The cycle life of each of the batteries is indicated as an index
when the cycle life of battery R3 is defined as 100.
TABLE-US-00002 TABLE 2 Initial charge- discharge A1/B2 efficiency
Cycle life T6 5% by mass 92% 102 R3 87% 100 T7 10% by mass 90% 45
R4 84% 42 T8 30% by mass 86% 36 R5 76% 32
[0089] As listed in Table 2, each of the batteries of the examples
had initial charge-discharge efficiency higher than and a cycle
characteristics comparable to or better than those of the batteries
of the comparative examples.
Example 9
[Production of Negative Electrode Active Material]
[0090] A Si powder (three nines (3 N), pulverized to 10 .mu.m) and
a Li.sub.2SiO.sub.3 powder (pulverized to 10 .mu.m) were mixed
together in a ratio by mass of 42:58 in an inert atmosphere and
charged into a pot (composed of stainless steel (SUS), volume: 500
mL) of a planetary ball mill (P-5, manufactured by Fritsch). Into
the pot, 24 balls (diameter: 20 mm) composed of stainless steel
(SUS) were charged. A lid was closed. Pulverization treatment was
performed at 200 rpm for 50 hours. Then the resulting powder was
removed in an inert atmosphere and subjected to heat treatment in
an inert gas atmosphere at 600.degree. C. for 4 hours. The
heat-treated powder (hereinafter, referred to as "base particles")
was pulverized. The pulverized powder was passed through a mesh
with 40-.mu.m openings, mixed with coal pitch (MCP250, manufactured
by JFE Chemical Corporation), and subjected to heat treatment in an
inert atmosphere at 800.degree. C. for 5 hours to coat surfaces of
the base particles with carbon, thereby forming conductive layers.
The coating weight of carbon is 5% by mass with respect to the
total mass of the particles including the base particles and the
conductive layers. The average particle diameter of the resulting
particles was adjusted to 5 .mu.m with a sieve to produce negative
electrode active material A9.
[Analysis of Negative Electrode Active Material]
[0091] SEM observation of the cross section of particles of
negative electrode active material A9 revealed that the Si
particles had an average particle diameter less than 100 nm and
that the Si particles were substantially uniformly dispersed in a
matrix composed of Li.sub.2SiO.sub.3. FIG. 3 illustrates an XRD
pattern of negative electrode active material A9. In the XRD
pattern of negative electrode active material A9, peaks originating
mainly from Si and Li.sub.2SiO.sub.3 were observed. A peak which
was observed at 2.theta.=about 27.0.degree. and which corresponded
to a plane with Miller indices (111) of Li.sub.2SiO.sub.3 had a
full width at half maximum of 0.233.degree.. A diffraction peak
originating from SiO.sub.2 was not observed at 2.theta.=25.degree..
Measurement of negative electrode active material A9 by Si-NMR
revealed that the content of SiO.sub.2 was less than 7% by mass
(equal to or lower than the minimum limit of detection).
[Production of Negative Electrode]
[0092] The negative electrode active material and polyacrylonitrile
(PAN) were mixed together in a ratio by mass of 95:5. After
addition of N-methyl-2-pyrrolidone (NMP), the resulting mixture was
stirred with a mixer ("AWATORI RENTARO" (Thinky Mixer),
manufactured by Thinky Corporation) to prepare a negative electrode
mixture slurry. The slurry was applied to a surface of copper foil
in such a manner that the mass of a negative electrode mixture
layer was 25 g per square meter. The resulting coating film was
dried in air at 105.degree. C. and subjected to rolling to produce
a negative electrode. The negative electrode mixture layer had a
packing density of 1.50 g/cm.sup.3.
[Preparation of Nonaqueous Electrolytic Solution]
[0093] LiPF.sub.6 was added to a solvent mixture of ethylene
carbonate (EC) and diethyl carbonate (DEC) mixed in a ratio by
volume of 3:7 to prepare a nonaqueous electrolytic solution having
a concentration of 1.0 mol/L.
[Production of Nonaqueous Electrolyte Secondary Battery]
[0094] Lithium metal foil and the negative electrode provided with
a Ni tab were arranged in an inert atmosphere so as to face each
other with a polyethylene separator interposed therebetween,
thereby forming an electrode assembly. The electrode assembly was
placed in a battery case formed of an aluminum laminated film. The
nonaqueous electrolytic solution was injected into the battery
case. The battery case was sealed, thereby producing battery
T9.
Example 10
[0095] Negative electrode active material A10 and battery T10 were
produced in the same way as in Example 1, except that the treatment
time with the ball mill was changed to 150 hours. In an XRD pattern
of negative electrode active material A10, a peak which was
observed at 2.theta.=about 27.0.degree. and which corresponded to a
plane with Miller indices (111) of Li.sub.2SiO.sub.3 had a full
width at half maximum of 0.401.degree..
Example 11
[0096] Negative electrode active material A11 and battery T11 were
produced in the same way as in Example 9, except that the treatment
time with the ball mill was changed to 20 hours. In an XRD pattern
of negative electrode active material A11, a peak which was
observed at 2.theta.=about 27.0.degree. and which corresponded to a
plane with Miller indices (111) of Li.sub.2SiO.sub.3 had a full
width at half maximum of 0.093.degree..
Example 12
[0097] Negative electrode active material A12 and battery T12 were
produced in the same way as in Example 9, except that the treatment
time with the ball mill was changed to 10 hours. In an XRD pattern
of negative electrode active material A12, a peak which was
observed at 2.theta.=about 27.0.degree. and which corresponded to a
plane with Miller indices (111) of Li.sub.2SiO.sub.3 had a full
width at half maximum of 0.051.degree..
Example 13
[0098] Negative electrode active material A13 and battery T13 were
produced in the same way as in Example 9, except that
Li.sub.2Si.sub.2O.sub.5 was used in place of Li.sub.2SiO.sub.3. In
an XRD pattern of negative electrode active material A13, a peak
which was observed at 2.theta.=about 24.9.degree. and which
corresponded to a plane with Miller indices (111) of
Li.sub.2Si.sub.2O.sub.5 had a full width at half maximum of
0.431.degree..
Example 14
[0099] Negative electrode active material A14 and battery T14 were
produced in the same way as in Example 13, except that the
treatment time with the ball mill was changed to 20 hours. In an
XRD pattern of negative electrode active material A14, a peak which
was observed at 2.theta.=about 24.9.degree. and which corresponded
to a plane with Miller indices (111) of Li.sub.2Si.sub.2O.sub.5 had
a full width at half maximum of 0.102.degree..
Example 15
[0100] Negative electrode active material A15 and battery T15 were
produced in the same way as in Example 9, except that the treatment
with the ball mill was performed at 150 rpm for 30 hours. In an XRD
pattern of negative electrode active material A15, a peak which was
observed at 2.theta.=about 27.0.degree. and which corresponded to a
plane with Miller indices (111) of Li.sub.2SiO.sub.3 had a full
width at half maximum of 0.192.degree.. The Si particles had an
average particle diameter less than 200 nm.
Comparative Example 6
[0101] A Si powder (three nines (3 N), pulverized to 10 .mu.m) and
a Li.sub.2SiO.sub.3 powder (pulverized to 10 .mu.m) were each
pulverized in an inert atmosphere for 50 hours with the ball mill
and then were mixed together in a ratio by mass of 42:58. The
resulting mixture was directly used as negative electrode active
material B6 without heat treatment. Battery R6 was produced in the
same way as in Example 1. In negative electrode active material B1,
while Li.sub.2SiO.sub.3 particles adhered to surfaces of the Si
particles, a Li.sub.2SiO.sub.3 matrix (continuous phase) was not
formed. That is, negative electrode active material B1 did not have
a composite particle structure in which the Si particles were
dispersed in the Li.sub.2SiO.sub.3 phase. In an XRD pattern of
negative electrode active material B6, A peak which was observed at
2.theta.=about 27.0.degree. and which corresponded to a plane with
Miller indices (111) of Li.sub.2SiO.sub.3 had a full width at half
maximum of 0.032.degree..
Comparative Example 7
[0102] Negative electrode active material B7 and battery R7 were
produced in the same way as in Example 19, except that the
treatment with the ball mill was performed at 50 rpm for 50 hours.
In an XRD pattern of negative electrode active material B7, a peak
which was observed at 2.theta.=about 27.0.degree. and which
corresponded to a plane with Miller indices (111) of
Li.sub.2SiO.sub.3 had a full width at half maximum of
0.042.degree..
Comparative Example 8
[0103] Negative electrode active material B8 and battery R8 were
produced in the same way as in Example 9, except that in the heat
treatment after the pulverization treatment with the ball mill, the
heat treatment was performed in an inert atmosphere at 1000.degree.
C. for 4 hours. In an XRD pattern of negative electrode active
material B8, a peak which was observed at 2.theta.=about
27.0.degree. and which corresponded to a plane with Miller indices
(111) of Li.sub.2SiO.sub.3 had a full width at half maximum of
0.038.degree..
[0104] Regarding the batteries of Examples 8 to 15 and Comparative
examples 6 to 8, the initial charge-discharge efficiency and the
appearance of the negative electrode active material particles were
evaluated by methods described below. Table 3 lists the evaluation
results.
[Initial Charge-Discharge Efficiency]
[0105] Charge
[0106] Constant-current charge was performed at a current of 0.2 It
until the voltage reached 0 V. Subsequently, constant-current
charge was performed at a current of 0.05 It until the voltage
reached 0 V.
[0107] Discharge
[0108] Constant-current discharge was performed at a current of 0.2
It until the voltage reached 1.0 V.
[0109] Interval
[0110] An interval between the charge and the discharge was 10
minutes.
[0111] The ratio of discharge capacity to charge capacity at the
first cycle was defined as initial charge-discharge efficiency.
Initial charge-discharge efficiency (%)=discharge capacity at first
cycle/charge capacity at first cycle.times.100
[Appearance Evaluation of Negative Electrode Active Material
Particles (Check for Particle Breakage)]
[0112] After one charge-discharge cycle of the batteries, the
batteries were disassembled in an inert atmosphere. The negative
electrodes were removed from the disassembled batteries. Cross
sections of the negative electrode active materials were exposed
with a cross-section polisher (manufactured by JEOL, Ltd.) in an
inert atmosphere. The cross sections were observed with a SEM to
examine the particles for the presence or absence of particle
breakage. A state in which two or more fine particles that had
originally been a single particle are present in the cross section
was defined as the particle breakage.
TABLE-US-00003 TABLE 3 Average Initial Presence or particle Full
width charge- absence of diameter Lithium at half Structure
discharge Uniformity particle of Si silicate maximum of particles
efficiency in particle breakage T9 <100 nm Li.sub.2SiO.sub.3
0.233 composite particles 79% uniform absent T10 <100 nm 0.401
82% uniform absent T11 <100 nm 0.093 73% uniform absent T12
<100 nm 0.051 65% uniform absent T13 <100 nm
Li.sub.2Si.sub.2O.sub.5 0.431 82% uniform absent T14 <100 nm
0.102 75% uniform absent T15 <200 nm Li.sub.2SiO.sub.3 0.192 81%
uniform absent R6 <100 nm 0.032 mixed particles 60% -- -- R7 500
nm 0.042 composite particles 61% nonuniform present R8 <100 nm
0.038 60% uniform absent
[0113] As listed in Table 3, in negative electrode active materials
A9 to A15 of the examples, the particle breakage is less likely to
be caused by charge and discharge, compared with negative electrode
active materials B6 to B8 of the comparative examples. Batteries T9
to T15 of the examples have good initial charge-discharge
efficiency, compared with batteries R6 to R8 of the comparative
examples. That is, when the negative electrode active material
containing the Si particles dispersed in the matrix composed of the
lithium silicate in which a diffraction peak corresponding to a
plane with Miller indices (111) has a full width at half maximum of
0.05.degree. or more is used, the initial charge-discharge
efficiency is improved, compared with the case where the negative
electrode active material in which the full width at half maximum
is less than 0.05.degree..
Example 16
[Production of Positive Electrode]
[0114] Lithium cobaltate, acetylene black (HS100, manufactured by
Denki Kagaku Kogyo K.K.), and polyvinylidene fluoride (PVdF) were
mixed together in a ratio by mass of 95:2.5:2.5. After
N-methyl-2-pyrrolidone (NMP) serving as a dispersion medium was
added to the resulting mixture, the mixture was stirred with a
mixer (T.K. HIVIS MIX, manufactured by PRIMIX Corporation) to
prepare a positive electrode mixture slurry. The positive electrode
mixture slurry was applied to aluminum foil, dried, subjected to
rolling with reduction rolls, thereby producing a positive
electrode including a positive electrode mixture layer formed on
each surface of the aluminum foil, the positive electrode mixture
layer having a density of 3.6 g/cm.sup.3.
[Production of Negative Electrode]
[0115] Negative electrode active material A9 used in Example 9 and
graphite were mixed together in a ratio by mass of 5:95. The
resulting mixture was used as negative electrode active material
A16 (negative electrode active material A9: 5% by mass). Negative
electrode active material A16, sodium carboxymethyl cellulose
(CMC-Na), and styrene-butadiene rubber (SBR) were mixed together in
a ratio by mass of 97.5:1.0:1.5. Water was added thereto. The
resulting mixture was stirred with a mixer (T.K. HIVIS MIX,
manufactured by PRIMIX Corporation) to prepare a negative electrode
mixture slurry. The slurry was applied to copper foil in such a
manner that the mass of a negative electrode mixture layer was 190
g per square meter. The resulting coating film was dried in air at
105.degree. C. and subjected to rolling to produce a negative
electrode including the negative electrode mixture layer formed on
each surface of the copper foil, the negative electrode mixture
layer having a density of 1.6 g/cm.sup.3.
[Production of Nonaqueous Electrolyte Secondary Battery]
[0116] Tabs were attached to the electrodes. The positive electrode
and the negative electrode equipped with the tabs were spirally
wound with a separator interposed therebetween in such a manner
that the tabs were located at the outermost peripheral portion. The
resulting electrode assembly was inserted into a case formed of an
aluminum laminated sheet. After vacuum drying was performed at
105.degree. C. for 2 hours, the nonaqueous electrolytic solution
was injected. An opening portion of the case was sealed, thereby
producing battery T16. The design capacity of the battery was 800
mAh.
Example 17
[0117] Negative electrode active material A17 and battery T17 were
produced in the same way as in Example 16, except that the amount
of negative electrode active material A9 added was changed to 10%
by mass.
Example 18
[0118] Negative electrode active material A18 and battery T18 were
produced in the same way as in Example 16, except that the amount
of negative electrode active material A9 added was changed to 30%
by mass.
Comparative Example 9
[0119] Negative electrode active material B9 and battery R9 were
produced in the same way as in Example 16, except that negative
electrode active material B6 used in Comparative example 6 was used
in place of negative electrode active material A9.
Comparative Example 10
[0120] Negative electrode active material B10 and battery R10 were
produced in the same way as in Comparative example 9, except that
the amount of negative electrode active material B6 added was
changed to 10% by mass.
Comparative Example 11
[0121] Negative electrode active material B11 and battery R11 were
produced in the same way as in Comparative example 9, except that
the amount of negative electrode active material B6 added was
changed to 30% by mass.
[0122] Regarding the batteries of Examples 16 to 18 and Comparative
examples 9 to 11, the initial charge-discharge efficiency and the
charge-discharge cycle characteristics were evaluated by methods
described below. Table 4 lists the evaluation results.
[Initial Charge-Discharge Efficiency]
[0123] Charge
[0124] Constant-current charge was performed at a current of 1 It
(800 mA) until the voltage reached 4.2 V. Subsequently,
voltage-constant charge was performed at a voltage of 4.2 V until
the current reached 1/20 It (40 mA).
[0125] Discharge
[0126] Constant-current discharge was performed at a current of 1
It (800 mA) until the voltage reached 2.75 V.
[0127] Interval
[0128] An interval between the charge and the discharge was 10
minutes.
[0129] The initial charge-discharge efficiency of each of the
batteries was measured under the charge-discharge conditions.
[Cycle Test]
[0130] Each of the batteries was subjected to a cycle test under
the charge-discharge conditions. The number of cycles required to
cause the discharge capacity to reach 80% of the discharge capacity
at the first cycle was measured and was defined as a cycle life.
The cycle life of each of the batteries is indicated as an index
when the cycle life of battery R3 is defined as 100.
TABLE-US-00004 TABLE 4 Initial charge- discharge A9 or B6
efficiency Cycle life T16 5% by mass 92% 105 R9 78% 100 T17 10% by
mass 89% 55 R10 72% 42 T18 30% by mass 81% 43 R11 65% 32
[0131] As listed in Table 4, each of batteries T16 to T18 of the
examples had high initial charge-discharge efficiency and good
cycle characteristics, compared with batteries R9 to R11 of the
comparative examples.
REFERENCE SIGNS LIST
[0132] 10 negative electrode active material particle, 11 lithium
silicate phase, 12 silicon particle, 13 base particle, 14
conductive layer
* * * * *